Only a handful of Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) holes penetrate more than 500 m into "normal" oceanic crust formed at mid-ocean ridges, and these are all, therefore, important reference holes. Among them, Holes 395A and 504B (Fig. 1) form the most important pair of reference sites for young, upper oceanic crust formed at slow and medium spreading rates, respectively. They are particularly imporant as reference sites for the hydrogeology of young oceanic crust, which has been studied with extensive downhole measurements and detailed heat-flow surveys at both sites (e.g., Fig. 2). Holes 395A and 504B are the best documented of several cases in which ocean bottom water is known to be flowing down open DSDP/ODP holes into permeable levels of upper basement. These examples suggest that young upper oceanic crust under a sediment cover is easily permeable enough to support active circulation of seawater, but we still barely understand the details of such off-axis hydrothermal circulation or its control by the pressure distribution and fine-scale permeability structure.
Site 395A is located in 7-m.y.-old crust, in an isolated sediment pond with low heat flow (Hussong et al., 1979) that might be considered somewhat typical of the structure and hydrogeological setting for thinly-sedimented crust formed at slow spreading rates. Since it was drilled in 1975-1976 (Melson, Rabinowitz, et al., 1979), Hole 395A has been revisited three times for an extensive set of downhole measurements: during DSDP Leg 78B in 1981 (Hyndman, Salisbury, et al., 1984), during ODP Leg 109 in 1986 (Bryan, Juteau, et al., 1988), and during the French wireline reentry campaign DIANAUT in 1989 (Gable et al., 1992). On each of these revisits, the first order of business was temperature logging in the hole long after it had reequilibrated from any prior disturbance by DSDP/ODP operations. Each of the three temperature logs showed strongly depressed borehole temperatures, essentially isothermal to a depth of about 300 m into basement (Becker et al., 1984; Kopietz et al., 1990; Gable et al., 1992). This indicates a strong downhole flow of ocean bottom water into the permeable upper basement, at rates of thousands of l/hr, virtually unabated over the two decades that the hole has been open (Fig. 3).
In comparison, temperatures measured during the multiple revisits to Hole 504B were initially strongly depressed to a depth of about 100 m into basement, but then rebounded nonmonotonically towards a conductive profile. This indicates that the rate of downhole flow has decayed since the hole was first drilled, and that the downhole flow is directed into a more restricted section of uppermost basement than in Hole 395A (Becker et al., 1983, 1985, 1989; Gable et al., 1989; Guerin et al., 1996). The comparison suggests that Hole 504B penetrates a more passive hydrothermal regime, whereas Hole 395A provides a man-made shunt into a more active circulation system in basement. The various observations at Site 395 generally support a model proposed by Langseth et al. (1984, 1992) for lateral circulation in the upper basement beneath the sediment pond where the site is located (Fig. 4), but we have little resolution on the details of such circulation.
A large proportion of holes drilled into young oceanic crust have proven to be drawing ocean bottom water down into permeable levels of basement (e.g., Erickson et al., 1975; Hyndman et al., 1976; Anderson and Zoback, 1982; Becker et al., 1983, 1984; Davis, Mottl, et al., 1992). Such downhole flow requires sufficient basement permeability and a differential pressure between the fluids in the borehole and the formation fluids. In general, we surmise that the necessary differential pressures may arise because of some combination of two independent effects:
(2) true, dynamically maintained underpressures due to active circulation in the basement that would occur even if the borehole were not present.